Many particle detectors for detecting biological agent aerosols, e.g., the Tier-3 Biological Aerosol Warning System, are based on the fact that biological materials fluoresce when irradiated with ultraviolet radiation (or light). Some conventional detectors have an optical cavity that defines a focal point within the optical cavity. The biological agent aerosol is directed at the focal point along with the ultraviolet light from an ultraviolet radiation (or light) emitter, such as a laser or an LED, and the ultraviolet light generates fluorescence from the biological agent aerosol at or near the focal point. However, problems can occur when using light emitters, e.g., semiconductor ultraviolet optical sources (SUVOSs), that have both a primary emission band with a center wavelength in the ultraviolet region (i.e., a primary emission band that is capable of eliciting a fluorescence response from a biological aerosol) and a secondary emission at longer wavelengths that overlaps and interferes with the fluorescence response. In particular, the secondary emission band can be scattered by particles in the aerosol detector's optical cavity, thereby creating a positive response signal in the aerosol detector regardless of whether the scattering particle was a biological molecule or not.
Filtering optics are sometimes located between the SUVOS and the focal point to attenuate radiation that is emitted in the secondary emission band. However, such filtering optics can make the distance between the SUVOS and the focal point undesirably long for use in some particle detectors and thus can make these detectors undesirably large and difficult to transport. Another problem is that setting up the filtering optics is often difficult and time consuming.